A user opens three browsers on the same PC to access www.cisco.com to search for certification course information. The Cisco web server sends a datagram as a reply to the request from one of the web browsers. Which information is used by the TCP/IP protocol stack in the PC to identify which of the three web browsers should receive the reply?

Question

BlackTom AI · Accepted Answer

TCP delivers data to the correct application by using port numbers; the destination port in an incoming TCP segment tells the OS which local socket (and thus which browser process) should receive the data.

1) Relevant knowledge points and concepts
- TCP/IP identifies endpoints with a 4-tuple: (source IP, source port, destination IP, destination port).
- A host running multiple applications (or multiple browser instances) uses different local (ephemeral) source ports for each outbound TCP connection.
- When the server replies, the packet’s destination port is the original ephemeral port the client used; the OS matches that destination port to the correct socket/process.

2) Step-by-step reasoning
- Each browser opens its own TCP connection to www.cisco.com using the server’s port 80 and a unique ephemeral source port on the PC (for example: 49152, 49153, 49154).
- The browser’s SYN has: src IP = PC, src port = ephemeral, dst IP = server, dst port = 80.
- The server replies with: src IP = server, src port = 80, dst IP = PC, dst port = that ephemeral port.
- When the PC receives the reply, the network stack checks the IP header (destination IP matches the PC) and then the TCP header’s destination port; that destination port identifies which socket (which browser instance) accepted the connection.

3) Example intermediate values
- Browser A used src port 49152 → server reply has dst port 49152.
- Browser B used src port 49153 → server reply has dst port 49153.
- The OS delivers the packet to the socket bound to the matching destination port.

4) Why the correct answer is correct
The TCP/IP stack uses the destination port on the incoming segment to look up the socket table and forward the data to the correct application instance.

5) Why other options are incorrect
- a. Destination IP address: same for all three browsers (the PC’s IP), so cannot distinguish processes.
- c. Source IP address: same server IP for all replies, not unique per browser.
- d. Source port number: typically the server’s port (e.g., 80) and identical for all replies, so it doesn’t identify which browser.

6) Conclusion
Therefore, the information used is the destination port number, which matches the provided correct answer.

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